1. Field of the Invention
The present invention relates to accumulators/compensators for pressurized fluent material lines (wherein the fluent material may be a gas, a liquid, a slurry or even a finely particulate material), such as may be found in heavy machinery, such as earth moving equipment, or other machines which incorporate hydraulic fluid lines or other pressurized lines.
2. The Prior Art
High pressure fluent material systems, for example hydraulic power transmission lines, brake fluid transmission lines and the like, typically operate in pressure ranges from a few hundreds to several thousands of pounds per square inch. Such fluent material transmission systems can be subjected from time to time to rapid pressure fluctuations which may be of substantial amplitude and frequency. An example of such fluctuations exists in the fluent material transmission systems associated with ABS braking systems which pulsate rapidly during hard braking.
In such high pressure fluent material systems, it is known to connect to the pressurized fluent material line a device called an accumulator or compensator. The function of the accumulator/compensator is to absorb temporary spikes in the line pressure toward evening out the pressure at the "end" of the pressurized fluent material line and to prevent possible damage to or blow-out of the pressurized fluent material transmission line.
The operation of any non-preloaded accumulator-compensator is that as soon as a pressure differential develops across the membrane or shell, some slight deflection of the membrane or shell will occur. Depending upon the stiffness of the system, the deflection may be so small, even under substantial pressure differentials, as to be virtually undetectable. Alternatively, if the system is not very stiff, measurable deflection may occur with even relatively small pressure differentials.
FIGS. 1 and 2 illustrate known prior art accumulator/compensator constructions.
In FIG. 1, a diaphragm- or bladder-type accumulator/compensator is shown. Diaphragm-type accumulator 32 typically would be connected by any suitable type of fluid connection to a pressurized fluent material line 33. Accumulator 32 typically comprised a shell 35, the interior space of which is substantially bisected by a flexible elastic diaphragm 37. Typically, diaphragm 37 would be manufactured from rubber, elastic, plastic, metal or the like. On the side of the diaphragm that is not exposed to the pressurized fluent material 38, air or other gas will be pumped in through inlet 39. A pressure gauge 40 may also be connected to space 38 in order to determine the static pressure of the air or other gas pumped into space 38. Typically, accumulator 32 will be calibrated so that during steady state operating conditions, space 36 will fill with the pressurized fluent material and the pressure of the gas in space 38 will be such that diaphragm 37 will be unextended as indicated by the solid line in FIG. 1. If a sudden spike in the pressure of the fluent material in line 33 is encountered, bladder 37 will distend and stretch to absorb the additional pressure force and fluent material volume, as indicated by the dotted line.
Such diaphragm type accumulators are typically suited only for relatively low pressure applications when constructed of metal and somewhat higher pressure applications (5000 psi) when constructed of rubber. That is, a metal diaphragm is not capable of accommodating deflection magnitudes and/or cycling required of higher pressure systems, before failure, while a rubber diaphragm (if backed by a pressurized gas), can withstand greater magnitude deflections and cycling. However, in the latter case the materials from which the diaphragms 37 are manufactured typically are somewhat porus relative to the fluent materials being transmitted in line 33 so that some leakage of the fluent material directly through the membrane may occur. Also, elastomeric natural or artificial rubber or plastic diaphragms sometimes may be subject to direct chemical attack by the fluent materials being transmitted in line 33.
FIG. 2 illustrates another known configuration of pressurized line accumulator/compensator. Accumulator 45, which may be connected to line 46 by any suitable fluid connection, incorporates a housing 48 (schematically illustrated as a simple box, but understood to have a configuration of any conventional design dictated by the requirements of the specific application). Housing 48 will include a metal bellows structure 50 closed at one end and opened at the other end to the connection to fluent material line 46, dividing the interior of housing 48 into two spaces, interior of bellows 51 and exterior of bellows 52. Typically, the open end of the bellows will be affixed to the housing surrounding the connection to the fluent material line in a sealing manner, e.g., welding, etc., so as to preclude escape of the pressurized fluent material from the interior region 51 to the exterior region 52. A biasing mechanism, represented in FIG. 2 schematically by spring symbol 54, may be connected to the movable end of bellows 50 in order to provide resistive biasing of the bellows in order to, for example, place a tensile or compressive pre-load on the bellows. In addition, the space 52 between the bellows 50 and the housing 48 may be filled with a pressurized gas in a manner similar to that described with respect to the prior art embodiment of FIG. 1.
The bellows used in the accumulator 45 of FIG. 2 have been typically fabricated with a specialized bellows construction, as shown in magnified detail in FIG. 3a.
Bellows 50 may be provided with convolutions 55, 56, 57, etc. (see FIG. 2). Each convolution, for example, convolution 55, may be formed from two contoured diaphragms 55' and 55". In order to give the bellows 50 sufficient strength and flexibility to undergo repeated cycles of expansion and compression, as a result of the forces exerted on the interior 51 by the pressurized fluent material and on the exterior 52 by the biasing mechanism 54 and/or the presence of any pressurized gas within space 52, bellows 50 is constructed from a plurality of discreet diaphragm members connected to one another by welding, etc. For example, as previously indicated, convolution 55 is comprised of diaphragm 55' and diaphragm 55". Welds are provided at the outer radially edges and inner radially edges of each diaphragm (designated by W in FIGS. 2 and 3a). The just described construction enables the bellows to accommodate relatively large steady state pressurized fluent material pressures (above 8000 psi typ.) and relatively large fluctuations of the pressure in the pressurized fluent material (0 to system max. psi, typ.) as well as relatively large magnitude cycles of compression and extension of the bellows itself. The pressure drop across the bellows is relatively small due to the pressurized gas in space 52. In addition, the particular cross sectional configuration of the bellows permits compression of the bellows diaphragms against one another so that the overall thickness of the compressed bellows is the actual thickness of the individual convolution diaphragms summed.
However, such prior art metal bellows accumulator constructions incur, of necessity, substantial costs in terms of actual material used as well as in the fabrication efforts required to manufacture such accumulators, including pressurized canisters as well as the specialized shaping of the individual convolutions and the requirement for welds at each connection between individual diaphragm members.
FIGS. 3b and 3c illustrate additional prior art methods, which have been used for reinforcing thin-walled bellows. FIG. 3b illustrates a thin-walled bellows which is unreinforced. The bellows may be defined by a variety of characteristics, including: the nominal diameter, d; the thickness, t; the amplitude of the convolutions, W; the length of the neck, I.sub.t ; support collar (for attachment to pipes) thickness, t.sub.c ; and wavelength of the convolutions, q. Generally, the thicker the bellows wall is, the stiffer it is and the greater the overall strength is. However, as the wall thickness increases, so does the susceptibility of the wall to high displacement bending fatigue. As the bellows undergoes cycling, through either extension and compression, or bending, localized buckling will ultimately occur with the time required in inverse proportion to the wall thickness. Accordingly, if a high displacement high cycling frequency must be accommodated, in order to reduce the wall thickness, the bellows must be reinforced, by reinforcing rings and/or equalizing rings, as shown in FIG. 3c.
However, such prior art metal bellows accumulator constructions incur, of necessity, substantial costs in terms of actual material used as well as in the fabrication efforts required to manufacture such bellows, including the specialized shaping of the rings and the requirement for forming the bellows with the rings on or splitting the rings for assembly. In addition, the relatively large thickness to diameter ratio (of thickened bellows), makes these bellows construction susceptible to fatigue from numerous extensions and compressions, and/or overly rigid and unresponsive to rapid system fluent material pressure fluctuations. Reinforced thin-walled bellows also suffer from the potential drawback that the reinforcement is not continuous, but is focused only in specific regions of the bellows. Such constructions are suitable for expansion joints where the reacting forces are predominantly axial. However, in accumulator/compensator applications, the predominant pressure forces act in all directions which requires continuous support along the membrane.
An oscillation damping construction is illustrated in Rohde, U.S. Pat. No. 5,575,262. The device of the Rohde '262 reference is a damper for the fuel circulation circuit of an internal combustion engine. A structure is positioned within the fuel distributor of a fuel injection system, which is fabricated from a resilient material, and has a plurality of gas-filled chambers. The chambers are positioned proximate the injector valves. Localized transient elevations in pressure of the fuel cause the gas-filled chambers to be temporarily compressed, to absorb and dampen such oscillations. The apparatus of the Rohde '262 reference is not configured for compensating for gross localized increases in fluent material volume; neither is it configured to provide an accumulator function, to act as a capacitor, to return stored energy to a fluent material system. Further, since only gas is used to support the resilient material, the apparatus of the Rohde '262 reference is not configured for high pressure systems or situations with large pressure differentials across the thickness of the resilient material.
It would be desirable to provide an accumulator/compensator construction that is capable of accommodating fluctuations in pressure in pressurized fluent material lines over a broad range of steady state operating pressures, especially high pressures.
It would also be desirable to provide a accumulator/compensator for pressurized fluent material systems which is capable of accommodating large variations present in a pressurized fluent material line while using relatively conventional constructions that are simpler and less expensive than prior art constructions for accumulator/compensators.
It would further be desirable to provide an accumulator/compensator which can handle relatively high pressures and/or high volume compensation.
Also, it would be desirable to provide an accumulator/compensator which is non-permeable and physically and/or chemically inert to the pressurized system fluent materials, and which can accommodate flowable materials such as liquids, gases and slurries.
These and other objects of the invention will become apparent in light of the present specification, claims and drawings.